Various types of antennae, including patch antennae, are employed with wireless-communication devices such as cell phones, hand-held personal digital assistant (PDA) devices, GPS receivers, laptop and tablet PCs, etc. Patch antennae are generally well suited for use with many such devices, in part because they have a low profile (i.e., height) and are relatively easy and inexpensive to manufacture. A typical patch antenna includes a radiating element that is used to both transmit and receive signals, and a ground plane. The radiating element and the ground plane are typically “patches,” i.e., substantially flat pieces of metal such as copper. The radiating element and the ground plane are generally disposed substantially parallel to each other, separated by a dielectric substrate disposed therebetween.
In general, the amount of electromagnetic power to be transmitted using a patch antenna and/or the strength of the signal to be received affect, in part, the size of the radiating element. The greater the power to be transmitted (or the weaker the signal to be received), the larger the required radiating element. However, if the radiating element is too large the antenna can become unsuitable for use with small devices such as cell phones or Bluetooth transceivers.
In designing antennae, typically two objectives are important. First, it is desirable to manufacture an antenna having a high efficiency. The efficiency of an antenna is the ratio of the power of a transmitted (i.e., radiated) signal to the input power, i.e., the power of the signal received for subsequent transmission. The second objective is to increase the gain of the antenna. The antenna gain is the ratio of the intensity of the radiation of the antenna in a desired direction to the intensity of radiation that would be produced by a hypothetical ideal antenna that radiates equally in all directions, and has no losses. Thus, the antenna gain relates to a fraction of the total power transmitted by the antenna in a desired direction. Other objectives in antenna design may include the desired frequency of transmission/reception and bandwidth of the antenna.
The size of the antenna's ground plane substantially affects the various antenna parameters described above, including the antenna's efficiency and gain. To achieve high efficiency (e.g., about 57%) and gain (e.g., about +5 dB), a typical ground plane is designed to be significantly larger than the radiating element, adding to the overall size of the antenna. For example, appliances such as cell phones, Bluetooth devices, and GPS receivers often employ an antenna that includes a radiating element of about 25 mm×25 mm. A typical ground plane used with such an antenna overlaps the radiating element and extends from each side of the radiating element by about 25 mm, so that the antenna's efficiency is about 57%. The distance by which the ground plane extends beyond the radiating element is called the “border.” Thus, the size of a typical antenna is about 75 mm×75 mm. The requirement for a large ground plane can make the communication device large and bulky, and, as described above, the antenna may be so large in some instances that it may become unsuitable for certain applications. On the other hand, a relatively small ground plane can decrease the antenna's efficiency and/or gain substantially, also making it unsuitable for certain applications.
One approach to addressing this problem is to introduce “defects” in the ground plane or to provide a cavity adjacent the ground plane. In a defected ground plane, a portion of the electrically conductive material (e.g., copper) comprised within the ground plane is removed from one or more locations, altering current distributions within the ground plane. This can mitigate current-crowding losses, and thus increase the antenna's efficiency. But, the removal of the conductive material permits radiation to be emitted through the defect, causing a reduction in the antenna's front-to-back-gain ratio. In other words, an antenna having a defected ground plane may transmit less radiation in a desired direction than an antenna of a similar size and structure, but having a defect-free (i.e., continuous) ground plane. For its part, the addition of a cavity often makes the antenna thicker, bulkier, and/or heavier.
Accordingly, there is a need for an improved antenna that can meet the multiple goals of small size, high efficiency, and high gain.